1. Field of the Invention
This invention relates to systems and methods for measuring radiation beam deviation in general, and particularly, in lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs), and other devices involving fine structures.
In some lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the substrate. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a semiconductor wafer). The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the mask's pattern, via imaging, onto a layer of radiation-sensitive material (photo-resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Instead of a mask, in some lithographic apparatus, the patterning device can be a patterning array that comprises one or more arrays of individually controllable elements. Sometimes, the pattern can be changed more efficiently in a maskless system compared to a mask-based system. These types of apparatus are referred to as Optical Maskless Lithographic (OML) apparatus.
Known lithographic apparatus include so-called steppers or step-and-repeat apparatus, and so-called scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a subsequent exposure. In a scanner, each target portion is irradiated by scanning the pattern through a beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a subsequent exposure.
In order to achieve optimum performance in a mask-based or OML apparatus, proper positioning and pointing of the illuminator beam is important. Conventionally, large optical systems (including lenses and mirrors) and detectors are used to measure deviation of the beam from its desired position and pointing. Typically, beam steering mirrors and other mechanisms adjust the beam to bring it back within an allowed range of position and pointing values, if the beam shifts outside the allowed range due to some reason. One such conventional large optical system is known as a Beam Measuring Unit (“BMU”). A BMU is not only large in size, it also has the additional limitation of wavelength dependence. A BMU designed for a particular actinic wavelength may not be suitable for another specific actinic wavelength or wavelength range. For example, a BMU designed for 248 nm actinic wavelength uses a 633 nm wavelength red laser for non-actinic measurement and calibration for practical advantages, because 633 nm is visible (248 nm is not) and requires less safety precautions. However, the same measurement and calibration data can not be used for an actinic wavelength of 193 nm, as refractive indices of lenses within the BMU are wavelength-dependent. Thus, significant design changes are needed for a BMU that would work for the 193 nm actinic wavelength.
There are additional limitations in the conventional beam deviation measurement systems. Usually, angular displacement sensors are located within the illuminator to measure beam deviation. Depending on the location of the angular displacement sensor within an illuminator, the effects of pulse polarization state, angular misalignments of detectors, and variation in laser pulse energy and wavelength may greatly affect the accuracy and precision of the calculated angular deviation of the beam. These effects become more significant for conventional beam deviation measurement systems, as those systems may not be positioned at an optimum location due to their large size. Moreover, the measurement range of conventional detectors is limited to portions of the entire required range. Most of the conventional detectors work better for larger deviations, but lose accuracy for smaller deviations. On the other hand, highly sensitive detectors that work well for the smallest deviations have zero or minimal sensitivity for larger deviations, because variations of the spread of angular deviation about a nominal deviation confound sensitive detectors that assume a collimated input.